Devil May Cry walkthrough/M16

Walkthrough part 1
When you start off, there is a Blue Orb just beside the levitating platform. There's nothing to see in the Coliseum anymore, so just leave by the blue double doors you came in through. Remember what was said about nothing being in the Coliseum? Well, forget it. Go back to start Secret Mission 9. Afterwards, in the courtyard of the garden, kill the Death Scythes and follow the lighted path back to the entry door.

Run up the spiral staircase and to the drawbridge, which is up and can't be used. Examine the glowing circle and use the Wheel of Destiny to lower it. Cross the bridge and enter the castle. Since it's nighttime, the entire place is covered in an eerie darkness.

In the main hall, grab the Blue Orb Fragments hanging about. The first one is beneath the right-hand side of the staircase. The second is hanging in mid-air between the knight statue and the second floor. To get it, jump on top of the Divinity Statue and perform a Stinger towards the knight statue. After you have those, whack the blue shield device where the big angelic statue used to be. (Where'd that go, anyway?) This will summon a bolt of lightning to strike the knight statue, transforming the knight into a... giant electrical bat? Don't question it; just kill it!

Plasmas have multiple shapes, and their attacks and strategies change depending on the form. As a bat, they largely fly around and try to zap you with a laser. It's pretty easy to dodge as long as you keep them in view. In the human form, they mirror Dante's attacks with Alastor, plus they can shoot a laser from their eye. Plasmas also have the nasty habit of splitting into three parts, however; the first one you face thankfully spares you that headache. Avoid using Air Raid! Alastor works but is incredibly weak against them on harder difficulties. Devil Trigger with Ifrit for best results.

Once that's dead, head through the blue doors into the airplane hangar. Here you encounter another Plasma, and this one splits into three. Do everything you can to take them out at the same time. If only one is left, it will split into three again. Kill it, and head through the red double doors to the left. There are some Marionettes and Fetishes here, so kill them. There's also a Blue Orb Fragment hiding in the ceiling here. Run away from the camera to the wall with the grotesque face above the painting, right next to the door to the Divinity Statue room. Jump and wall hike into the darkness of the rafters. You may need to move around a bit to find just the right spot to get it to fall.

Make your way down the hall to the Cathedral, breaking the suits of armor along the way. Examine the ominous puddle of blackness to start the boss fight.

Boss: Nightmare
Nightmare is a nightmare! Then again, it has to be to have a weapon designed after it. Nightmare is completely invulnerable to attacks when the lights are off, and only shows its vulnerable core when iluminated. The key to defeating it is to keep the place lit and keep moving.

Basically, Nightmare has two forms: puddle, and solid(ish). In his puddle form, it never shows its core, and has the ability to engulf Dante and drag him into its belly, where Dante must fight and defeat one of the other bosses in order to escape.

Attacks

 * Small arms fire - Little orbs will float around Nightmare and pepper Dante with streams of projectiles. Keep moving.
 * Lance - A spike will protrude out and try to skewer Dante. Keep moving to avoid it.
 * Slugs - Strange worm-like things will crawl to Dante and wrap around him. They explode, dealing damage. Shoot, dodge, or hit them to get rid of them.
 * Core vent - When being exposed, a white light will eminate from the core and damage Dante if he gets too close. This goes away after a short time, and Dante can attack it afterwards.
 * Light blast - Nightmare charges a powerful beam of light and fires it at Dante. Keep to its rear to avoid this.
 * Homing missiles - Missiles spawn from Nightmare's exposed core and home in on Dante. Run away, jump and dodge, or use Devil Trigger to shrug off the attacks and attack the core.
 * DT Absorption - Nightmare will drain Dante's Devil Trigger gauge.
 * Swallow whole - Only when in puddle form, Nightmare will cover Dante and pull him into a different dimension to fight another boss. In order to excape, Dante must first defeat a number of Sargasso, then either Phantom or Griffon at full strength.
 * See Mission 3 and Mission 9 for strategies against Phantom and Griffon.

Strategy
The most important thing to do is to keep the place lit! There are a number of shield devices arranged around the walls; keep them as full as possible. Only when the place is lit will Nightmare attain its solidus form and expose its core occasionally. After this, keep moving. Many of its attacks are highly damaging but slow.

Most of Nightmare's attacks require it to expose its core, which will appear either on its backside (if it can be called that) or high on its back. Keep your wits about you and get on top of the thing when you see the white, glowing core. Using Devil Trigger is highly recommended, as it allows you to deal extra damage to the core while ignoring the stagger effects of its little minion orbs' pot shots.

Walkthrough part 2
There is another Secret Mission you can attempt here. Exit into the hallway (the way you came in) and run all the way back to the airplane hangar. This starts Secret Mission 9: Shadow of Darkness.

With Nightmare defeated, exit through the large doors to the Broken Bridge. Get across it some way, whether that's jumping over the pieces, falling through and dealing with the Sargasso, or using Air Raid.

On the far side, if you did not collect any of the small Red Orbs scattered around the outside of this place, you're in for a treat. Those little orbs have grown up and gotten MUCH bigger! Patience pays off!

When you've collected everything, examine the snake and staff relief and use the Staff of Hermes. Jump through the left image into the gatekeeper's room to end the mission. If you'd like, you can jump through to the bedroom first to pick up a Blue Orb Fragment and an Untouchable, but you will be coming back to this room later, so it is probably better to not waste your time.